The invention relates to a method for producing polycrystalline silicon rods having a large diameter, in which disks composed of a material having a lower electrical resistivity than the polycrystalline silicon are introduced in order to avoid cracking and chipping in the silicon rod.
During the deposition of polysilicon according to the Siemens process, high-purity elemental silicon is deposited from the gas phase on the surface of silicon rods. In this case, in a deposition reactor, from a mixture of hydrogen and halosilanes or a hydrogen-containing silicon compound, elemental silicon is deposited from the gas phase on the surface of a thin silicon rod heated to 900 to 1200° C.
In this case, the silicon rods are held in the reactor by specific electrodes, which generally consist of high-purity electrographite. In each case two thin rods having a different voltage polarity at the electrode mounts are connected at the other thin rod end by means of a bridge to form a closed electric circuit. Electrical energy for heating the thin rods is fed via the electrodes and the electrode mounts thereof. A mixture of hydrogen and halosilanes is added via inlet nozzles at the baseplate of the deposition reactor. In this case, the halosilanes decompose at the surface of the thin rods. In this case, the diameter of the thin rods increases. At the same time, the electrode grows, starting at its tip, into the rod foot of the silicon rods. After a desired setpoint diameter of the silicon rods has been attained, the deposition process is ended, and the glowing silicon rods are cooled and demounted.
A particular importance is accorded here to the material and the shape of the electrodes. They serve, firstly, for retaining the thin rods, for transferring the current flow into the silicon rod, but also for transferring heat and also as a secure stage for the growing rod in the reactor. Since the trend is toward ever longer and heavier rods and the rod pairs, which in the meantime can have a weight of hundreds of kilograms, are only anchored by means of the electrodes in the reactor, precisely the choice of the shape and of the material constitution is very important.
Electrodes according to the prior art consist of a cylindrical base body in the lower part and a conical tip in the upper part. A hole for receiving the thin rod is provided at the conical tip. In this case, the lower end of the electrode is placed into a metallic electrode mount, via which the current is fed in. Such electrodes are generally known and are used for silicon deposition for example in U.S. Pat. No. 5,284,640.
Graphite is principally used as material for the electrodes since graphite is available with very high purity and is chemically inert under deposition conditions. Furthermore, graphite has a very low electrical resistivity.
After the deposition process, the obtained U-shaped rod pairs composed of polysilicon are cut to length on the electrode side and on the bridge side. The rods obtained have to be free of cracks and break-offs at both rod ends and over the entire rod length. Afterward, the rods thus obtained are cut to length to form rod pieces, where it is necessary to comply with customer requirements such as rod length and rod weight. These rods also have to be free of cracks and chipping-off on both sides and over the entire rod length.
What is disadvantageous about all the electrodes known from the prior art is that said electrodes, at the transition between electrode and the silicon rod or in the silicon rod in the vicinity of the electrode, tend to a greater or lesser extent to cracking or to chipping-off of the material and thus make the silicon rod unstable.
In order that a high yield of crack-free rod length is obtained, the electrode- and bridge-side rod ends of the obtained U-shaped rod pairs composed of polysilicon are intended to have cracks and chipping-off to the least possible extent, and ideally not at all. Rod regions having cracks signify a high outlay when cutting the rods to length, since the rod ends are cut to length in slices until freedom from cracks is reached.
Length, diameter and weight of the processed polysilicon rods are part of the customer specification. The customer requirements are shifting further and further toward long and thick rods. On the other hand, cracks and break-offs increase as the deposition diameter increases during production. A method for avoiding cracks therefore has high economic potential.
There are already various publications for avoiding cracks and chipping-off in rods composed of polycrystalline silicon.
U.S. Pat. No. 6,676,916 describes, for example, a method in which small flaws such as holes or notches are provided in the thin rods below the bridge. Further possibilities mentioned include thickening of the thin rod by compression or constriction of the conduction cross-section. At these defects, a cleavage plane is intended to be formed during the deposition as a result of disrupted crystalline growth. In the event of thermal strains, these planes are then intended to act as preferred fracture locations.
Thick and crack-free polysilicon rods are used in production processes such as, for example, the floating zone process, the recharging of Czochralski processes or the cutting of new thin rods. These processes presuppose a smooth rod surface and a compact rod cross-section without flaws and without regions having different crystal structures in the rod. Therefore, a uniform crystal structure of the polysilicon rods is necessary during the deposition process. Small flaws on the thin rod, such as are proposed in U.S. Pat. No. 6,676,916, completely grow together during such deposition processes even in the case of a thin rod diameter. As a result, the region no longer acts as a preferred fracture location in the case of thick rods.
JP-63074909 discloses a method for avoiding cracks and chipping-off in which the silicon rods are heated by high-frequency AC current. With high-frequency AC current, the current density is shifted toward the rod edge according to the so-called skin effect. The temperature difference between rod center and rod surface can thereby be kept small. The higher the frequency, the greater the shift in the current density to the rod edge. In order to achieve an appreciable effect, frequencies of >100 kHz are necessary. What is disadvantageous about this method is that in conjunction with the high current intensities and voltages necessary for heating the rods, a very costly shielding of power supply and deposition installation becomes necessary. Under practical and economic conditions, therefore, this method can be implemented only with difficulty.
No method known from the prior art made it possible, with the large rod diameters customary nowadays, to provide economic and simple methods that prevent cracking and the occurrence of chipping-off in the region of the transition to the rod foot and to the rod bridge.
Consequently, the object was to provide a simple method for producing silicon rods having a large diameter, which method brings about a reduction of cracks and break-offs both at the bridge-side and at the electrode-side rod ends and thus increases the crack-free rod length after the rod ends have been cut to length.
Surprisingly, it has now been found that, by incorporating disks above the electrodes used and below the bridge between the two rods of a rod pair, said disks consisting of a material having a lower electrical resistivity than the polycrystalline silicon under deposition conditions, the rod length free of cracks and free of chipping-off can be significantly increased.